Protective coatings are applied to the interior of metal food and beverage containers (e.g. cans) to prevent the contents from coming into contact with the metal surfaces of the containers. Contact of the container contents with the interior metal surface, especially where acidic products such as soft drinks and tomato juice are involved, can lead to corrosion of the metal container and result in contamination and deterioration of the contents. Protective coatings are also applied to the interior of food and beverage containers to prevent corrosion in the headspace of the container between the fill line of the food product and the container lid, which is particularly problematic with high salt content food products.
Metal container interiors are typically coated with a thin thermoset film to protect the interior metal surface from its contents. Various synthetic (co)polymer compositions and their blends, including poly(vinyl chloride) (co)polymers; vinyl-functional (meth)acrylic (co)polymers; polybutadiene (co)polymers; phenol-formaldehyde (co)polymers; epoxy-functional (co)polymers; alkyd/aminoplast resins and oleoresinous materials; have been used as interior can protective coatings. These heat-curable (co)polymer compositions are usually applied as solutions or dispersions in volatile organic solvents.
The heat-cured protective coating compositions generally should exhibit sufficient adhesion and flexibility to maintain film integrity during container fabrication. Sufficient coating adhesion and flexibility also are needed for the coating composition to withstand processing conditions the container is subject to during product packaging. Other desired performance features of the cured coatings include corrosion protection and sufficient chemical, abrasion and mar resistance. The coatings used on the interior of metal food containers preferably also meet government regulatory criteria. For food contact, the (co)polymeric and materials used in these coatings are typically derived from components acceptable to the U.S. Food and Drug Administration (FDA) as published in Title 21 of the Code of Federal Regulations, § 175.300.
Multi-coat coating systems recently have been used to coat the interior of food and beverage containers, wherein the over-coat or lacquer contains an epoxy resin cross-linked with a phenolic resin. Such epoxy-based “Gold lacquers” typically exhibit good adhesion and are suitable for storing acidic foodstuffs and beverages. However, there is a perception that some epoxy-based coatings, containing mobile Bisphenol A (BPA), Bisphenol F (BPF), and aromatic glycidyl ether compounds, are less acceptable for foodstuffs storage.
Currently, the food packaging industry and consumer groups are seeking coated metal packaging articles prepared from coating compositions free from mobile BPA, BPF, and aromatic glycidyl ether compounds and exhibiting excellent corrosion and chemical resistance, acceptable adhesion and flexibility during container fabrication. The art continues to seek an ideal coating composition for use as a protective coating for metal foodstuffs containers.